In mobile telecommunications networks, fading is a well known problem which affects transmitted signals. A signal received in a mobile radio system experiences fading due to multiple reflective paths between the transmitter and receiver. This multi-path fading can result in errors in the received data.
Fading results from the presence of reflectors in the environment surrounding a transmitter and receiver, which creates multiple paths that a transmitted signal can traverse. As a result, the receiver sees the superposition of multiple copies of the transmitted signal, each traversing a different path. Each signal copy will experience differences in attenuation, delay and phase shift while travelling from the source to the receiver. This can produce either constructive or destructive interference, amplifying or attenuating the signal power seen at the receiver.
In this regard, two types of fading can be defined, namely frequency selective fading and flat fading (i.e. frequency non-selective). These forms of fading depend on the duration of a transmitted data symbol relative to the delay spread of the paths.
Flat fading occurs when the duration of the transmitted data symbol is large compared to the relative delay of the paths. Therefore, all frequency components of the signal will experience the same magnitude of fading. Flat fading can be addressed using techniques such as error coding, simple equalization or adaptive bit loading.
Alternatively, if the duration of the transmitted data symbol is small compared to the relative path delay the fading is classed as frequency selective. In frequency-selective fading, the coherence bandwidth, which measures the separation in frequency after which two signals will experience uncorrelated fading, of the channel is smaller than the bandwidth of the signal. Different frequency components of the signal therefore experience de-correlated fading. By comparison, in flat fading, the coherence bandwidth of the channel is larger than the bandwidth of the signal.
Fading can cause poor performance in a communication system because it can result in a loss of signal power without reducing the power of the noise. This signal loss can be over some or all of the signal bandwidth.
The effects of fading can be combated by using a “diversity” scheme to transmit the signal over multiple channels that experience independent fading, and thereafter coherently combining them at the receiver. The probability of experiencing a fade in this composite channel is then proportional to the probability that all the component channels simultaneously experience a fade, a much more unlikely event. Diversity schemes vary the transmitted signal by a given transmission characteristic, and can be achieved in time, frequency and/or space.
OFDM (Orthogonal Frequency Division Modulation) is an example of a spread spectrum technique which employs frequency diversity to provide enhanced robustness to fading. OFDM divides the wideband signal into many slowly modulated narrowband sub-carriers, each exposed to flat fading rather than frequency selective fading. The flat fading can then be combated using error coding, simple equalization or adaptive bit loading.
Another approach is to use a rake receiver. Rake receivers are radio receivers designed to counter the effects of multipath fading. They do this by using several “sub-receivers” each assigned to a different multi-path component. Each sub-receiver independently decodes a single multi-path component, which are delayed copies of the original transmitted wave travelling through a different echo path, each with a different magnitude and time-of-arrival at the receiver. Since each component contains the original information, if the magnitude and time-of-arrival (phase) of each component is computed at the receiver (through a process called channel estimation), then all the components can be added coherently to improve the reliability of the transmitted information. Rake receivers are common in a wide variety of CDMA and W-CDMA radio devices such as mobile phones and wireless LAN equipment.
However, with such spread spectrum techniques, if the delay spread is small, the signal is no longer frequency selective and the link may no longer be optimal. In such situations time diversity can be employed, typically termed Time Delay Transmit Diversity (TDTD). In TDTD multiple transmit antennas are used to artificially create time dispersion by transmitting replica signals with different relative delays from the different antennas. By exploiting the independent levels of fading, it is possible to recover a significant amount of any lost bit error-rate (BER) performance and improve overall system performance by using TDTD.
FIG. 2 illustrates this technique for a two antenna system. In this Figure, a base station (BS) transmits a signal s(t) towards a mobile terminal (MS). This signal is transmitted via two antennas. The first antenna transmits with signal s(t) with no time delay, whilst the second antenna transmits the signal with a time delay ΔT, so that the signal transmitted from the second antenna is s(t−ΔT).
The relative delay T is fixed, and chosen to create a frequency selective channel over the transmitted signal bandwidth. For instance, a typical fixed value would be chosen at deployment and based on some average delay spread appropriate for at least the whole cell area for the BS, as well as the antenna configuration (e.g. taking into account tilt, azimuth etc).
The integration of the fixed time delay is transparent to the MS and the BS, as the receiver of the MS considers the delay as being due to normal multi-path reflections in the propagation channel and accordingly will utilize the inherent properties to optimize signal reception. TDTD therefore does not therefore require any standards support. Further, it can be offered as a third party add-on hardware solution, placed between the output transmission port of the BS and the additional antenna(s).
Whilst this approach of introducing a fixed time delay into one or more additional signal paths can work well, there is room for further improvement.
For instance, the TDTD technique described above assumes a mobile station experiences the same conditions at all locations within the serving base station's cell. Whilst this can be a reasonable approximation in some circumstances, it is of course not necessarily the case, and can lead to some instances of fading not being adequately corrected or accounted for.